Young Men’s Pagan Association. Refers our world’s YMCA youth hostels. YMCA stands for ‘Young Men’s Christian Association’, and is often made fun of (e.g. Monthy Python and their ‘Young Men’s Anti-Christian Association’). The YMCA runs summer courses for children, and presumably for adults as well.
In our world the YMCA somehow became associated with the homosexual scene (I think quite a few people singing merrily along to the Village People’s disco hit ‘YMCA’ would have been very surprised to learn what the song was really about), hence the “studded collars and oiled muscles” bit.